Southern Alaska contains economically important mineral deposits that formed during unique episodes of volcanism and magma emplacement some 70 to 50 million years ago. Deposits of copper, zinc, silver, lead, gold, molybdenum and other metals are predicted to form along the margins of large igneous intrusions and along fault zones that cross-cut the igneous rocks but the origin of many of these deposits is not known. This project will document the compositions of igneous rocks and develop models for their origins which will provide a system for predicting the distribution of mineral resources in the northern Talkeetna Mountains region of southern Alaska. This outcome is consistent with a national strategy to 'ensure availability of reliable geologic, geochemical, geophysical, and mineral locality data for the United States' as described by the U.S. Geological Survey Mineral Resources Program. This project will also provide state-of-the-art scientific training for undergraduate students, including international collaboration.
The focus of this project will be on 62 to 50 million year old volcanic and plutonic rocks that were emplaced in a terrane suture zone during the final stages of terrane collision. Preliminary radiometric age and geochemical data reveal that these rocks are not typical of continental margin arc systems. The three-year work plan for this project is to produce new geochronologic, geochemical, and field mapping data on these igneous rocks to address the following fundamental questions: What were the source(s) of magmas during and following terrane collision? Can subduction-related arc rocks be distinguished geochemically from post-subduction (collisional) igneous rocks in a systematic manner? Was magmatism related to collisional and/or post-collisional deformation? These questions will be evaluated using major and trace element chemistry (XRF and ICPMS methods); whole rock Nd, Sr, Pb, and Hf isotope and single zircon Hf isotope chemistry (all by LA-MC-ICPMS); and 40Ar/39Ar and zircon U/Pb dating in collaboration with Dr. Sun-Lin Chung at National Taiwan University in Taipei. Field mapping, sample collection, and data synthesis will be completed by the PI and undergraduate students from Allegheny College. The age, geochemical, and isotopic results of this project will be disseminated through NSF-funded web-based databases (NAVDAT, EarthChem) and will provide a basis for assessing the distribution of economic mineral deposits as well as contribute to the long-term goal of establishing well-constrained models for Cenozoic magmatism across southern Alaska.
The focus of this project was on 80 to 40 million year old igneous rocks that are exposed in the Talkeetna Mountains of south-central Alaska. In the field of geology, there are well-established models for the origin of igneous rocks that form along the main types of plate tectonic boundaries around the world. For example, in the case where an oceanic plate converges with the edge of a continent, the oceanic plate can be pushed beneath the continent and magmas can be produced above the down going plate. This results in a predictable distribution and composition of magmas and volcanoes along the continental margin (e.g., an ‘arc’ such as the chain of Cascade volcanoes in the Pacific Northwest or the Aleutian volcanoes in southwestern Alaska). While this type of magmatism has been common in the geologic past of southern Alaska, the igneous rocks of this project do not conform to this end-member model. Instead, they were formed in a setting of collision between a fragment of oceanic plateau (thickened oceanic crust) and the former margin of southern Alaska. This project produced new geologic mapping, geochronologic, and geochemical data on these igneous rocks to determine the source(s) of magmas during and following collision and to evaluate whether these types of igneous rocks have chemical compositions that distinguish them from more typical arc magmas. These questions have so far been evaluated using major and trace element chemistry and zircon U-Pb radiometric dating; these analyses were completed in a collaboration at National Taiwan University where there are state-of-the-art lab facilities. This analytical work was provided as part of a scientific collaboration at no cost to the proposed project. Forthcoming lab work for this project, through 2015, will include isotope analyses to complement the data obtained so far. These data confirm that granite and related volcanic rocks in the Talkeetna Mountains that range from 63 to 56 million years, do have unique chemical compositions to distinguish them from typical continental margin arc rocks. Magmas to form these igneous rocks were derived in part by melting sedimentary crustal rocks along the zone of collision. The rising of heat and mantle material from deep beneath the crust contributed to the formation of magmas and crustal melting. Some of the granites are cross-cut by narrow (10’s cm to meters wide) but very coarse-grained granitic intrusions that contain garnet. These coarse intrusions are similar to some ore-bearing deposits in other parts of Alaska. As such, one motivation for this project was the fact that other parts of southern Alaska contain economically important mineral deposits that formed in association with the same age igneous rocks as this study. Often times, metallic deposits are found in and adjacent to large bodies of granite; when exposed at the surface, these deposits can be eroded into streams. The U.S. Geological Survey has compiled the chemical composition of stream sediment throughout Alaska and these data for the project study area show relatively high concentrations of rare earth elements, beryllium, and tin. As part of this project, these stream sediment data were compared with igneous rock composition in an attempt to identify high concentrations of these elements in any particular rock unit. While locally high concentrations of silver were found along small-scale fault zones, the results of this study did not reveal any notable mineral deposits directly within the plutonic rocks (e.g., no rare earth element deposits or metallic veins). As such, the Late Cretaceous and Paleocene igneous rocks of this study are not likely to have strategic economic potential. Economic resources in the project area are more likely to be found distributed in the adjacent sedimentary rocks that were intruded by the plutons. This project provided scientific training for eight undergraduate students, including collecting and analyzing geologic field data and state-of-the-art laboratory data, the latter as part of an international collaboration. This investment in scientific training is commensurate with a current national priority for increasing and retaining the number of scientists in the U.S. and with promoting globalization among the U.S. science community.
